The composition of exhaust produced by the combustion of hydrocarbon fuels is a complex mixture of oxide gases (NOX, SOX, CO2, CO, H2O), unburned hydrocarbons, and oxygen. Measurement of the concentration of these individual exhaust gas constituents in real time can assist in improved combustion efficiency and lower emissions of polluting gases. Various devices have been proposed to operate as exhaust gas sensors that have the capability of measuring the concentration of a gas constituent in an exhaust stream. Some of these devices includes a ceramic sensing element encased in a housing and positioned in an exhaust stream. A part of the exhaust stream is directed into the housing to measure the concentration of some of the constituents of the exhaust. The ceramic sensing element may include sensing regions and heating elements configured to heat the sensing regions to a desired temperature range. The sensing region may measure the concentration of particular exhaust constituent with acceptable accuracy when the temperature of the sensing region is within the desired temperature range. A catalyst may also be used to condition the exhaust gases to increase the sensitivity of the sensor. During operation, a part of the exhaust from an engine may be directed over the sensing regions through the catalyst. The heating elements in the sensing element may heat the sensing regions and the catalyst to a high temperature to increase the accuracy and sensitivity of the measurements.
U.S. Pat. No. 7,217,355 (the '355 patent) issued to Nair et al. on May 15, 2007 discloses one such sensor. The sensor of the '355 patent includes a NOx sensor, catalyst, and a heating rod enclosed in a housing. The sensor of the '355 patent is configured to measure the concentration of NOx in an exhaust stream. The catalyst conditions the exhaust gases before being directed over the NOx sensor. The heating rod heats the NOx sensor and the catalyst, to increase the sensitivity of the sensor. This heat may also be conducted to the housing. In an application where it is desired to position such a sensor in an exhaust stream, the relatively cooler exhaust stream flowing over the housing may remove heat from the housing and thereby cool the sensor. This cooling of the sensor by the exhaust stream may compromise the accuracy of the sensor measurements. The disclosed sensor assembly is directed at overcoming shortcomings as discussed above and/or other shortcomings in existing technology.